1. Field of the Invention
The present invention relates to surface-acoustic-wave-device mount substrates for mounting a surface-acoustic-wave device on a surface thereof, and used as high-frequency modules to be mounted to personal digital assistants such as mobile phones, and relates to high-frequency modules and communication apparatus.
2. Description of the Related Art
In general, multilayer ceramic substrates in which relatively high-density wiring is possible are frequently used as insulating substrates, and high-frequency modules mounted with, e.g., semiconductor devices and surface-acoustic-wave devices (hereinafter, sometimes referred to as SAW chips). The high-frequency modules that use the multilayer ceramic substrates include a multilayer ceramic substrate made of alumina or glass ceramic and a distribution metal conductor made of tungsten, molybdenum, copper, or silver on the surface. Examples of the high-frequency modules include those in which semiconductor devices or SAW chips are tightly sealed with covers or organic resin.
Specifically, as shown in FIG. 14, the SAW chip 62 has a comb electrode 64, at least a pair of input and output terminals 65a and 65b, and a ring-shaped ground terminal 66 around the comb electrode 64 and the input and output terminals 65 bonded to the back of a piezoelectric substrate 63. On the other hand, the SAW chip 62 has input and output electrodes 68a and 68b and a ring-shaped ground electrode 69 bonded to the surface of the insulating substrate 67. The input and output terminals 65a and 65b and the ring-shaped ground terminal 66 are bonded to the input and output electrodes 68a and 68b and the ring-shaped ground electrode 69 on the surface of the insulating substrate 67 with conductive resin or are bonded with a conductive adhesive 70 such as solder. Thus, the SAW chip is mounted on the surface of an insulating substrate 67, so that this assembly is used as a duplexer or the like.
In general, the ring-shaped ground electrode 69 and the input and output electrodes 68a and 68k are electrically connected to a grounding conductor pattern 72 and input and output back electrodes 73 on the back of the insulated substrate 67 through via-conductors 61 disposed in the insulating substrate 67.
With such amount structure, the ring-shaped ground terminal 66 and the ring-shaped ground electrode 69 bonded with the conductive resin 70 serve as a sealer and as such, the comb electrode 64, the input and output terminals 65a and 65b, and the input and output electrodes 68a and 68b are tightly sealed.
Therefore, high coplanarity is required for the surface of insulating substrates to ensure performance stability, packaging reliability, and sealing reliability of the SAW chips. At the same time, the accuracy of dimension in the X- and Y-axes directions, that is, in plane directions, of insulating substrates is also required with an increasing packing density of semiconductor devices to be mounted on substrate surfaces.
Known multilayer ceramic substrates are manufactured in such a way that through holes are formed in a green sheet made from alumina or glass ceramic, into which conductor metal paste made of tungsten, molybdenum, copper, or silver is poured to form via-conductors, and the conductor paste is printed on the surface of the sheet, and the sheet is layered and fired. However, this method is not capable of minute dimension control because of shrinkage characteristic of ceramic in the directions of X- and Y-axes, so that the method cannot be applied to insulating substrates for ICs with high packing density and a plurality of pins.
As a method for solving the problems, a technique of manufacturing insulating substrates with high dimension accuracy is proposed in which the layered ceramic green sheets are fired under pressure, or an inorganic compound layer that is not sintered at the firing temperature is formed on the surface of the green sheets, and they are fired at the same time so as to shrink only in the direction of Z axis, that is, in the direction of thickness, and to prevent shrinkage in the directions of X- and Y-axes, thereby maintaining the initial dimensions (for example, Japanese Unexamined Patent Publication JP-A 7-86743 and JP-A 2001-339166).
However, by the method of manufacture in which the shrinkage characteristic in the directions of X- and Y-axes is reduced, the substrate shrinks greatly in the direction of Z-axis. Thus, as shown in FIG. 15, when the input and output electrodes 58 and the ring-shaped ground electrode 59, which are connected to the SAW chip 62, are formed on the surface of the insulating substrate 67, and when the via-conductors 71 are formed directly under the input and output electrodes 68a and 68b and the ring-shaped ground electrode 69 and directly connected therewith, the electrodes 68a, 68b and 69 may protrude in the direction of thickness because sintering temperature and shrinkage behavior are different between the region of the via-conductors and the region without the via-conductors. Thus, the electrodes 68a, 68b and 69 directly on the via-conductors 71 have a tendency to rise extremely. Therefore, high-frequency modules mounted with the SAW chips as shown in FIG. 14 have generated packaging failure and imperfect sealing due to the unevenness.